The invention relates to a sensor element for detecting a physical measurement variable such as a pressure, a temperature, a capacitance, or a gap width between two bodies that move in relation to each other during operation and experience high tribological stress.
Lubricated contacts or elastohydrodynamic contacts of the kind that occur, for example, in bearings (ball bearings, roller bearings, or slide bearings) or other elements coupled to each other, such as gear contacts, cam-tappet contacts, etc., are subjected to very high tribological stresses. Under such operating conditions, the lubricating film between the bodies that move in relation to each other can become locally insufficient or can be broken, i.e. a so-called “mixed friction” occurs, which causes the load capacity limits of the materials used to be very quickly reached or even exceeded and causes damage to the corresponding components. In this respect, it is important to monitor the contact temperatures, contact pressures, and/or contact stresses in such highly stressed lubricated component contacts during operation in order to at least detect—e.g. by testing damage mechanisms—but ideally to prevent, the occurrence of critical loads, for example as a result of high friction energy densities, which, when exceeded, cause components, for example, of an injection system, to fail due to adhesion (“seizing” in automotive engineering).
Since contact points are subjected to very high tribological stresses, primarily when there is mixed friction, the sensor elements used must also be able to withstand very high mechanical and tribological stresses. On the other hand, they should exert as little influence as possible on the measurement variables or friction conditions to be detected.
A sensor element for detecting temperature or pressure between two bodies that move in relation to each other and experience high tribological stress is known from H. Peeken and A. Köhler, Konstruktion 32 (June 1980), pp. 241–246 “Modern Metrology by means of Vapor-Deposited Transducers in Slide and Roller Bearings” [Moderne Messtechnik mittels aufgedampfter Geber in Gleitund Wätlzlagern]. This sensor element makes use of the fact that in many materials, the electrical resistance depends on both the temperature and the pressure so that after a calibration measurement, the pressure and/or temperature can be determined simply by measuring the resistance.
In particular in the above-mentioned publication, metal layers are deposited onto the components to be tested; these metal layers are designed and positioned so that measurement signal is picked up at the desired location. It is also necessary that the actual sensor segment and the contact surfaces or strip conductors connected to it be electrically insulated from the component underneath them, which is normally metallic. To this end, a glass layer or ceramic layer, for example a silicon oxide layer or an aluminum oxide layer, is produced as an insulating intermediary layer between the sensor segment and the component that is provided with it. In addition, Cr2O3 layers are also already known as insulating intermediary layers.
The disadvantage of the known insulation layers between the sensor segment and the component that is provided with it lies in the fact that due to their differing coefficients of thermal expansion, in many cases they adhere poorly for example to metals, a problem which frequently cannot be satisfactorily solved, even with an additional layer of adhesion promoter. Another disadvantage lies in the fact that previously known insulation layers are relatively brittle and are insufficiently able to withstand tribological stress.
The object of the current invention was to produce a sensor element for detecting a physical measurement variable such as the pressure or temperature, between two bodies that move in relation to each other during operation and experience high tribological stress, where the actual sensor segment or in general, the actual sensitive layer, is separated from the body underneath it by an insulation layer, which assures a sufficient electrical insulation and is also able to withstand very high tribological stresses. In this connection, an insulation layer with sufficient electrical insulating properties is understood to be a layer whose electrical conductivity does not necessarily have to be zero, but is significantly lower than the electrical conductivity of the sensitive layer and is preferably so low as to be negligible.